Silicon carbide fiber currently put into practical use includes silicon carbide fiber ("NICALON" produced by NIPPON CARBON) and titanium-containing silicon carbide fiber ("TYRANNO fiber" produced by UBE INDUSTRIES). These silicon carbide fibers are produced by rendering the precursor polycarbosilane fiber or polytitanocarbosilane fiber infusible by thermal oxidation, followed by calcination in an inert atmosphere (the treatment for rendering a precursor infusible will hereinafter be referred to as infusibilization). However, a large quantity of oxygen is introduced into the precursor during thermal oxidation for infusibilization. For example, NICALON and TYRANNO fibers have an oxygen content of about 10% by weight and about 18% by weight, respectively. Because of the high oxygen content, both and fiber undergo thermal decomposition when exposed to high temperatures exceeding about 1500K and suffer an abrupt reduction of strength. It is reported that the maximum strength of NICALON and TYRANNO fiber at temperatures below the respective thermal decomposition temperature is about 3 GPa and about 3.5 GPa, respectively.
In an attempt to improve the heat resistance of silicon carbide fiber, it has been proposed to conduct the infusibilization by means of an ionizing radiation to obtain a silicon carbide fiber having a reduced oxygen content and thereby having a thermal decomposition temperature of about 1800K or higher. The silicon carbide fiber obtained by such a process has a breaking strength of about 2.5 GPa even after heat treatment at about 1800K. It has also been proposed to conduct the ionizing irradiation for infusibilization in a reactive gas, such as acetylene, so as to reduce the requisite radiation dose. This technique is directed to increase the efficiency of radiation-induced crosslinking.
Taking advantage of infusibilization by irradiation, a process for infusibilizing silicon carbide fiber by radiation-induced oxidation to improve the strength has been developed. With this process, silicon carbide fiber having a breaking strength of about 5 GPa at the maximum can be produced. However, since the process involves introduction of oxygen into the silicon carbide fiber, the heat resistance of the resulting fiber is equal to or slightly higher than that reached by infusibilization by thermal oxidation.